1. Field of the Invention
This invention relates to a multilayer, light-sensitive article having (a) a layer containing a photopolymerizable material containing a photopolymerization initiator and a polymerizable ethylenically unsaturated compound, and (b) a layer containing a thermoplastic resin. More specifically, the invention relates to multilayer, light-sensitive articles containing halomethyl-1,3,5-triazine photopolymerization initiators that resist migration from the layer of photopolymerizable material to layers adjacent thereto.
2. Description of the Prior Art
Light-sensitive articles comprising a photopolymerizable layer containing a polymerizable ethylenically unsaturated compound and a photopolymerization initiator are well known in the graphic arts. The purpose of the photopolymerization initiator, alternatively referred to herein as "photoinitiator", is to generate free radicals upon imagewise exposure of the polymerizable layer to actinic radiation to initiate polymerization of the ethylenically unsaturated compound to produce an image. One common method of developing such an image utilizes the solubility differential created between the material in less soluble (exposed) region and the material in the more soluble (unexposed) region by removal of the latter material with an appropriate solvent. Another common method of developing such an image utilizes a laminate in which the material in the photopolymerized (exposed) region preferentially adheres to one of the layers in the laminate and can be separated from the material in the unpolymerized (unexposed) region by peeling the laminate apart. A general review of photopolymerization and development techniques is described in C. G. Roffey, Photopolymerization of Surface Coatings, John Wiley and Sons (New York, 1982), Chapters 4 and 6.
Light-sensitive articles have been employed in the preparation of overlay or surprint color proofing sheets, which are used to verify the accuracy, with respect to color, of separation films used in the preparation of printing plates. For example, a single-sheet, four-color proof (surprint) must duplicate with great accuracy the colored image and background to be achieved on a printing press using printing plates made with the same separation films. Processes and methods of color proofing are described in M. H. Bruno, Principles of Color Proofing, Gama Communications (Salem, N.H., 1986).
Whenever a layer of a photopolymerizable material that is a component of a proofing sheet construction comes into contact with a layer of resin, it is possible for one or more colored components of the layer of photopolymerizable material to migrate into the layer of resin and bring about a discoloration in that layer. For example, U.S. Pat. No. 4,596,757 describes a process that utilizes a photographic element having a substrate and sequentially carried thereon three layers which are a release layer, a colored photohardenable layer, and an adhesive layer. The process of the patent allows the operator to have a choice of two image formation methods using the same composite photographic element.
These choices are full colored layer transfer with subsequent sequential image formation, and individual image formation with image only superimposition.
1. If an image only transfer is desired, the operator may expose the composite through a negative flat placed in contact, emulsion to adhesive. The resulting latent image film is then transferred to a temporary receiving sheet, preferably a polyester film, before developing the image with the developer solution. Each color in the four color transfer process is likewise exposed and transferred to its own polyester sheet and the image developed. As a last step, the image may be transferred in register to give a right reading image.
2. If a full solid color layer transfer is preferred, a first coated composite is transferred to a permanent base and is then exposed with subsequent development of the image. This is a sequential color transfer process and each of the subsequent colors must be transferred on top of the previous color, exposed in register and developed to obtain the full four-color proof.
In either method, one or more adhesive layers remain superimposed in the background (i.e., the unexposed and developed areas), the particular number of layers depending on the number of color images superimposed on the receptor sheet. The migration of one or more components of the layer of photopolymerizable material into the contiguous resin layer generally results in a discolored background and an unsatisfactory proof.
Other examples of proofing constructions in which the layer of photopolymerizable material comes into contact with a layer of resin are described in U.S. Pat. Nos. 3,884,693; 4,174,216; 4,806,451; and 4,923,780.
Examples of light-sensitive compounds that initiate photopolymerization by decomposing to generate free radicals upon exposure to light are well known and have been described in C. G. Roffey, Photopolymerization of Surface Coatings, John Wiley and Sons (New York, 1982), Chapter 3. Chromophore-substituted halomethyl-1,3,5-triazine compounds are a particularly effective class of photopolymerization initiators. These triazine compounds respond to radiation in the near ultraviolet to visible light region. For example, U.S. Pat. Nos. 3,954,475; 3,987,037; 4,189,323; 4,696,888; 4,772,534; 4,826,753; and 4,837,128 disclose such chromophore-substituted halomethyl-1,3,5-triazine compounds. However, in the multilayer, light-sensitive articles previously described, these compounds were found to migrate out of the layer of photopolymerizable material to produce a discoloration in the background layer. U.S. Pat. No. 4,933,452 discloses triazine derivatives of polymeric or oligomeric compounds having a reduced tendency for migration. Unlike chromophore-substituted triazine compounds, these triazine compounds require that a separate sensitizing dye be incorporated in the layer of photopolymerizable material to be responsive to the radiation of the light sources used in graphic arts. The sensitizing dye used in the examples of U.S. Pat. No. 4,933,452, ethyl Michler's ketone, is known to migrate out of the layer of photopolymerizable material into a contiguous resin layer and cause a discoloration of the background.
Accordingly, it would be desirable to develop halomethyl-1,3,5-triazine compounds that resist migration from the layer of photopolymerizable material of a multilayer, light-sensitive article in order to overcome the problem of discoloration of the background.